1. Field of the Invention
This invention relates to a surface potentiometer for measuring the surface potential of an object to be measured, and more particularly to a surface potentiometer for measuring the surface potential of a recording medium such as a photosensitive medium in an electrostatic recording apparatus such as an electrophotographic copying machine or the like.
2. Description of the Prior Art
Among such surface potentiometers, there is one type in which, as already disclosed in Applicant's U.S. Pat. No. 4,261,660, issued on Apr. 14, 1982 a cylindrical chopper having a small hole in the cylinder portion thereof is fixed as one shield means to the output shaft of a compact DC motor and rotated. In the surface potentiometer using such cylindrical chopper, by intermittently shielding the small hole of the cylindrical chopper between the measured surface of a measured portion and a measuring electrode disposed at opposed positions, an AC voltage having an amplitude proportional to the differential voltage between the measured surface and the electrode is induced on the electrode.
However, such a conventional surface potentiometer requires a motor of high accuracy in order to obtain a stable AC signal of high frequency and this has been a cause of high cost. Also, in the conventional surface potentiometer, when the voltage applied to the motor fluctuates, the frequency of the AC signal put out fluctuates and therefore, such a special contrivance has been necessary that the fluctuation of the voltage applied to the motor is small for the fluctuation of the power source voltage. Further, the use of the motor has caused the size of the potentiometer to be limited by the motor and a difficulty has been encountered in making the potentiometer compact.